Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, and more particularly to an image forming apparatus such as a copier, a facsimile machine, a printer, a plotter or a multi-functional system including a combination thereof, and more particularly to an image forming apparatus using a toner other than process colors such as yellow, magenta, cyan, and black.
Description of the Related Art
Market demand has grown for an ability to add extra values to output images in on-demand printing. In order to satisfy such market demand, there have known color image forming apparatuses capable of producing images using a special color toner. The image forming apparatuses of this type are normally equipped with at least one image forming unit for the special color toner in addition to image forming units for the process colors.
There are various kinds of special color toners: a colorless, transparent toner also known as a clear toner, a white toner, an auxiliary color toner to support a color that is difficult to reproduce with process colors. For example, in order to enhance color development of a color image, a solid white image is formed on an entire surface of colored paper as a base with a white toner, or an uppermost layer of an image surface is formed with the white toner when forming an image on a transparent film based on an assumption that the transparent film is viewed from the back. The use of the special color toner is different from the conventional use of color toners.
In the image forming apparatuses equipped with the special color toner, upon printing an image with the superimposed special color toner, depending on the type of recording media, a toner image is not transferred properly at a secondary transfer portion, resulting in a transfer failure such as a partial toner transfer failure known as toner dropouts (blank spots).
It is known that in general, a highly pressurized toner upon transfer process causes toner dropouts. Prior to the transfer process, the toner layer is formed in a state in which there is a space between toner particles. Upon transfer, pressure is applied to the toner particles, thereby packing the toner particles together and increasing adhesion between the packed toner and a photoconductor (in secondary transfer process, adhesion between the toner and an intermediate transfer belt). As a result, the toner cannot be transferred to the intermediate transfer belt (in the secondary transfer process, to a recording medium), which results in toner dropouts even when a transfer electric field is applied.
Toner dropouts tend to occur easily with a large amount of toner (hereinafter referred to as a toner adhesion amount). Furthermore, toner dropouts occur more easily at a periphery portion of a fine line image and a solid image, the toner adhesion amount of which tends to be excessive due to a so-called edge effect in development. Furthermore, in addition to color toners in yellow, magenta, cyan, and black, when printing with the special color toner, the toner adhesion amount of a toner image formed on the intermediate transfer belt tends to be greater than when printing a normal color image. As a result, toner dropouts occur easily during secondary transfer process.
It is known that when the relation of friction coefficients μ of a photoconductor (Pμ), an intermediate transfer belt (Iμ), and a recording medium (Rμ) satisfies Pμ<Iμ<Rμ, suppression of toner dropouts tends to be more significant than that using paper having a high smoothness and a low friction coefficient μ such as gloss coated paper. The relation of friction coefficients of the gloss coated paper is degraded, and hence the toner image is not transferred properly, resulting in toner dropouts.